1. Field of the Invention
The present invention relates to a process for producing polyamides which are useful as molding materials or packaging materials. More particularly, the invention relates to a process for the production of polyamides by melt-polymerization, which is capable of providing a desired polyamide at a high efficiency with batch-to-batch stability in quality. In the process, the molar ratio of a diamine component (total of the monomer remaining not polymerized and the monomer polymerized to constitute the polyamide) and a dicarboxylic acid component (total of the monomer remaining not polymerized and the monomer polymerized to constitute the polyamide) in a reaction mixture (hereinafter occasionally referred to merely as xe2x80x9cmolar ratioxe2x80x9d or xe2x80x9ccharged molar ratioxe2x80x9d) and physical properties such as molecular weight are rapidly measured by a near-infrared spectrometer during the polymerization operation, and the molar ratio and the physical properties are automatically and rapidly controlled on the basis of the obtained results of the near-infrared measurements.
2. Description of the Prior Art
Polyamide have been generally produced by a dehydration polycondensation of a diamine monomer and a dicarboxylic acid monomer. Of the polycondensation methods, usually employed is a melt-polymerization method which is conducted at a temperature not less than the melting point of the polyamide being produced. In this production method, it is important to maintain reaction conditions such as a molar ratio of monomers and a reaction time at setpoints in order to ensure the production of polyamides with batch-to-batch uniformity and stability in their quality. Therefore, various techniques have been employed to maintain the reaction conditions such as a molar ratio of monomers, a reaction time, a reaction temperature and a reaction pressure at setpoints.
However, it is usually difficult to constantly maintain the reaction conditions at the setpoints since the reaction conditions are varied depending on the performance of a reactor and the internal and external factors. Therefore, the change with time of some properties of polyamide such as molar ratio, molecular weight, relative viscosity and melt viscosity is measured during the polymerization process in order to adequately control the polymerization reaction as well as in order to accurately determine the timing for changing the polymerization conditions and the end point of the polymerization reaction.
Hitherto, the properties of polyamide during the polymerization process have been measured by several different analyzing methods. For instance, the number-average molecular weight of polyamide is calculated from a carboxyl end group concentration and an amino end group concentration thereof which are measured by a neutralization titration of a solution of the polyamide in a specific solvent. The relative viscosity of the polyamide is determined by comparing the dropping speed by second of a solution of the polyamide in a concentrated sulfuric acid with that of only the concentrated sulfuric acid, each dropping speed being measured by using a viscometer.
For measuring the properties of polyamide with the passage of time during the polymerization process by the above analyzing methods, sampling of a reaction mixture from a melt-polymerization apparatus is inevitably required. The sampling procedure, however, is not an appropriate method, because it is time-consuming, it requires a plurality of sampling numbers, and it is one of the external factors which disturb the polymerization process. In addition, the time required until obtaining the results of the analysis from the sampling is usually 2-4 hours or longer. Therefore, the real-time measurement of properties of polyamide during the polymerization process is actually impossible, thereby failing to rapidly control the polymerization.
When the melt-polymerization is further continued in another melt-polymerization apparatus, it is necessary to know the properties of the polyamide produced in the previous melt-polymerization apparatus. However, only the viscosity data have been hitherto available. A melt-polymerized polymer is generally subjected to a solid-polymerization to increase its molecular weight. To adequately determine the solid-polymerization conditions, the properties of the melt-polymerized polymer are necessary to be known. However, the melt-polymerized polymer should be stored in a silo, etc. until the results of the analysis are obtained, thereby reducing the production efficiency.
Japanese Patent Publication No. 48-36957 proposes to use a viscometer for a real-time measurement of properties of polyamide during the continuous polymerization process. However, in this method, only a melt viscosity is measured by the viscometer and the other properties such as molar ratio of charged monomers and end group concentrations cannot be measured, resulting in insufficient control of the polymerization.
In recent years, there has been proposed an on-line measurement of the properties of polyesters, etc. using a near-infrared spectrometer during the production thereof. Near-infrared radiation is more permeable as compared to ultraviolet radiation and infrared radiation, and therefore, very suitable for non-destructive analysis and real-time analysis. However, the near-infrared spectroscopy was not hitherto put into practice because of various problems in the stability of light source, the spectroscopic system, the detector, and the hardware and software of computers for processing spectral data. With recent development of related techniques, near-infrared spectrometers solved in many of these problems have become commercially available.
Each of Japanese Patent Applications Laid-Open Nos. 2-306937, 10-182802, 11-60711 and 11-315137 discloses to measure various properties of polyester using a near-infrared spectrometer during the production thereof, and to control the polymerization conditions on the basis of the measured values. However, none of these prior art references describe or discuss a process for controlling the polymerization conditions for producing high molecular weight polyamides by measuring properties of polyamide using a near-infrared spectrometer during its production.
Japanese Patent Application Laid-Open No. 6-322054 discloses a process for controlling the production of phenol resins by using a near-infrared spectrometer to measure the amounts of the compositions in a reaction system and carrying out the reaction while monitoring the degree of progress of the reaction on the basis of the measured results. However, this prior art reference also fail to describe or discuss a method of controlling polymerization conditions for the production of polyamide.
U.S. Pat. No. 5,573,952 discloses a process for measuring the concentration of a solution comprising an amide solvent and aramid polymers using a near-infrared spectrometer to adjust the amount of the solvent. However, the polymerization disclosed therein is solution polymerization, and nothing is described or discussed therein about the measurement in a melt-polymerization process. In addition, in the proposed process, only the polymer concentration in the solution is measured, and there is no teaching about the measurement of properties of polyamide itself.
U.S. Pat. No. 5,674,974 discloses a continuous process for the production of polyamides by melt-polymerization and a process control method using a near-infrared spectrometer. In the proposed process, the carboxyl end group concentration and the amino end group concentration are measured, and the balance thereof is controlled by varying the feed amount of diamine on the basis of the measured results, thereby producing an aimed polyamide and preventing formation of solids in a polymerization apparatus.
However, the measurement using the near-infrared spectrometer actually taught therein is limited only to the production of polyamide from adipic acid and hexamethylenediamine, and there is no description and discussion therein about the measurement using the near-infrared spectrometer in the production of polyamide from other diamines, e.g., xylylenediamine and bisaminomethylcyclohexane. U.S. Pat. No. 5,674,974 further fails to teach or describe a process for increasing the molecular weight in another polymerization apparatus after melt-polymerization.
Also, U.S. Pat. No. 5,674,974 relates only to a continuous melt-polymerization conducted in a specific multi-stage reactor using a near-infrared spectrometer, and describes nothing about a batch-wise melt-polymerization. In addition, the patent is completely silent about the direct near-infrared measurement of properties other than the end group concentrations of polyamide, e.g., molar ratio of monomers, molecular weight, relative viscosity and melt viscosity.
An object of the present invention is to provide a process for producing a high-molecular polyamide with a batch-to-batch stability in its quality at a high production efficiency by rapidly measuring (A) a molar ratio of a charged diamine monomer and a charged dicarboxylic acid monomer in a reaction mixture, and (B) at least one property selected from the group consisting of a water content of the reaction mixture, and a carboxyl end group concentration, an amino end group concentration, a molecular weight, a relative viscosity and a melt viscosity of polyamide in the reaction mixture using a near-infrared spectrometer during the polymerization process; and automatically and rapidly controlling the polycondensation reaction on the basis of the measured values.
As a result of extensive researches in view of the above objects, the inventors have found that a high-molecular polyamide with a batch-to-batch stability in its quality is produced by rapidly and accurately measuring properties of the polyamide being produced using a near-infrared spectrometer during the melt-polymerization process, and rapidly and accurately controlling the melt-polymerization conditions on the basis of the measured values. The present invention has been accomplished on the basis of this finding.
Thus, the present invention provides a process for producing a polyamide constituted of a dicarboxylic acid unit comprising 80% by mol or more of an aliphatic dicarboxylic acid unit having 6 to 12 carbon atoms and a diamine unit comprising 80% by mol or more of xylylenediamine unit or bisaminomethylcyclohexane unit, the process comprising (1) carrying out a melt-polymerization in batch-wise manner by adding a diamine monomer directly into a molten dicarboxylic acid monomer, thereby producing a polyamide; (2) measuring (A) a molar ratio of a diamine component and a dicarboxylic acid component in a reaction mixture, and (B) at least one property selected from the group consisting of a water content of the reaction mixture, and a carboxyl end group concentration, an amino end group concentration, a molecular weight, a relative viscosity and a melt viscosity of the polyamide obtained in the step (1) using a near-infrared spectrometer when a number average molecular weight of the polyamide obtained in the step (1) reaches 6000 to 20,000; and (3) regulating the molar ratio to a desired level and determining a time for further continuing the melt-polymerization on the basis of measured values (A) and (B), while continuing the melt-polymerization.
The present invention further provides a process for producing a polyamide constituted of a dicarboxylic acid unit comprising 80% by mol or more of an aliphatic dicarboxylic acid unit having 6 to 12 carbon atoms and a diamine unit comprising 80% by mol or more of xylylenediamine unit or bisaminomethylcyclohexane unit, the process comprising (1) carrying out a melt-polymerization in batch-wise manner by adding a diamine monomer directly into a molten dicarboxylic acid monomer, thereby producing a polyamide; (2) measuring (A) a molar ratio of a diamine component and a dicarboxylic acid component in a reaction mixture, and (B) at least one property selected from the group consisting of a water content of the reaction mixture, and a carboxyl end group concentration, an amino end group concentration, a molecular weight, a relative viscosity and a melt viscosity of the polyamide obtained in the step (1) using a near-infrared spectrometer when a number average molecular weight of the polyamide obtained in the step (1) reaches 6000 to 20,000; (3) regulating the molar ratio to a desired level or determining a time for further continuing the melt-polymerization on the basis of measured values (A) and (B), while continuing the melt-polymerization; (4) repeating the measurements (A) and (B) just before discharging a polyamide obtained in the step (3) from a melt-polymerization apparatus; and (5) controlling polymerization conditions of a subsequent melt-polymerization on the basis of measured valued (A) and (B) obtained in the step (4).
The present invention still further provides a process for producing a polyamide constituted of a dicarboxylic acid unit comprising 80% by mol or more of an aliphatic dicarboxylic acid unit having 6 to 12 carbon atoms and a diamine unit comprising 80% by mol or more of xylylenediamine unit or bisaminomethylcyclohexane unit, the process comprising (1) carrying out a melt-polymerization in batch-wise manner by adding a diamine monomer directly into a molten dicarboxylic acid monomer, thereby producing a polyamide; (2) measuring (A) a molar ratio of a diamine component and a dicarboxylic acid component in a reaction mixture, and (B) at least one property selected from the group consisting of a water content of the reaction mixture, and a carboxyl end group concentration, an amino end group concentration, a molecular weight, a relative viscosity and a melt viscosity of the polyamide obtained in the step (1) using a near-infrared spectrometer when a number average molecular weight of the polyamide obtained in the step (1) reaches 6000 to 20,000; (3) regulating the molar ratio to a desired level or determining a time for further continuing the melt-polymerization on the basis of measured values (A) and (B), while continuing the melt-polymerization; (4) repeating the measurements (A) and (B) just before discharging a polyamide obtained in the step (3) from a melt-polymerization apparatus, and solidifying the polyamide obtained in the step (3) by cooling; and (5) controlling polymerization conditions of a subsequent solid-phase polymerization on the basis of measured valued (A) and (B) obtained in the step (4).